A major effort will be to use protons, deuterons and 3He particles, in the track segment method, to produce defined LET values up to 150 keV/micron. Synchronized Chinese hamster cells, at several phases of the cell cycle, will be used to determine the reaction between cell killing and LET. In parallel experiments, the production of chromosome aberrations will be studied as a function of LET. Using the hamster embryo system, an attempt will be made to discover the factors which lead to the striking difference in transformation frequency according to whether cells are irradiated in utero or in situ. With the T10 1/2 cells, the transformation frequency will be determined for gamma rays in protracted exposures at low dose rate, and also for alpha particles. BIBLIOGRAPHIC REFERENCES: Rossi, H.H. "A note on the effects of fractionation of high LET radiations." Radiation Res., Vol. 66, pp. 170-173, 1976. Rossi, H.H. "Interrelation between physical and biological effects of small radiation doses." Reprint from Biological and Environmental Effects of Low-Level Radiation, Vol. 1. IAEA-SM-202/509, pp. 245-251, Vienna, 1976.